This invention relates to submergible electric pump motors and is particularly concerned with increasing the operating life of such motors by improvements in the insulation system for the stator windings.
For many years submergible pump units have been employed in the pumping of oil or water from wells. Typically, the submergible pump unit comprises an electric motor and a centrifugal pump suspended colinearly in the well by tubing or cable, with the entire pump unit submerged in the liquid to be pumped. The submergible pump motor is normally filled with an oil that serves to lubricate moving parts, insulate electrical parts, cool the motor, and exclude the ambient well fluid from the interior of the motor. Some means, such as a protector containing oil, accommodates the thermal expansion and contraction of the oil resulting from the running and non-running cycle of the motor. Representative submergible pump units are disclosed, for example, in U.S. Pat. Nos. 1,951,919, 1,970,484; 2,001,649; 2,236,887; 2,251,816; 2,270,666; 2,315,917; 3,672,795, all assigned to the same assignee as the present invention.
Submergible electric pump motors employed to pump from great depths may be 50 feet in length, for example, and may be stacked to provide the motive power required to drive an equally lengthy pump. Such pump motors are not inexpensive, and when they fail in service, the pump unit must be pulled from the well--an expensive procedure. Accordingly, sophisticated insulation systems are employed in the pump motors to minimize electrical breakdowns. For example, the stator structure of one type of submergible motor sold by the assignee of the present invention includes insulating sleeves at top and bottom, insulating laminations at opposite ends of the steel laminations, insulating slot liner tubing for the windings, multiple layers of insulation on the winding conductors themselves, insulating tape on the end turns, insulating sleeves on the connection to the end turns, and an insulating varnish that impregnates the entire internal stator structure. The materials of which the various insulating components are composed are selected to provide long life, according to the data provided by the manufacturers of the insulation, and from this data one would expect that the submergible motors would operate for long periods of time free of electrical breakdowns. Unfortunately, such is not the case. Indeed, electrical insulation failures in submergible motors greatly reduce the useful life of such motors in practice.
Failures of submergible electric pump motors are to be expected, for submergible pump units are often called upon to operate in harsh and hostile environments under varying pressure, temperature, and other ambient conditions. Well environments vary widely and are not always predictable. Nevertheless, with sophisticated insulation systems and oil-filled interiors, submergible electric pump motors should be free of electrical breakdowns for much longer periods than has proven to be the case.
When submergible pump motors fail electrically, the ultimate failure is commonly a power arc, which may melt part of the stator. After the failure, it is difficult to ascertain precisely what happened to cause the failure. Moreover, because of the diverse and unpredictable environments in which submergible motors operate, there has been no ready solution to the electrical failure problem. The present invention solves that problem.